In an image reader such as a copy machine, a scanner or the like, photoelectric conversion is conducted by an image processor installing a charge coupled device (CCD). Recently, a sample-and-hold circuit is provided in the image processor to stabilize a level of an output signal for corresponding to highly increasing of reading speed of the image reader. In the stabilization, an output signal of the CCD is performed to be held in a sampling process.
In such the situation, a pre-buffer having smaller driving force is provided at a latter part of the sample-and-hold circuit to stabilize the sampling step during higher driving. Accordingly, a constitution of two-step buffer in which a main buffer with larger driving force is driven by the pre-buffer, is provided.
When the copy machine, the scanner or the like using such the image processor is provided as series products, sample-and-hold function may be not necessary dependent on a kind of the products.
In such a case, an output data of the CCD is directly conducted to input into the pre-buffer where the sample-and-hold circuit is set to be usually in the sampling state.
When the pre-buffer is constituted with one conductive-type MOS transistors, the output level is lower than the input level.
A problem is conventionally generated in a case that the sample-and-hold function is not necessary. Namely sensitivity of the image processor is lowered as the signal is passed through the pre-buffer. Further, electrical current consumption due to the pre-buffer is also remained a significant problem.